1. Field of the Invention
The present invention relates to coating compositions based on polyurethanes from which water vapor-permeable coatings can be produced.
2. Description of the Prior Art
It is known to prepare vapor-permeable coatings by physical means. In this method the polyurethane, dissolved in a solvent, is applied in the form of a layer to a substrate or to a release-substrate and introduced in the wet state into a bath that contains a non-solvent for the polymer which is miscible with the solvent. The non-solvent penetrates into the solution layer where it gradually precipitates and coagulates the polyurethane. During the drying of the resultant solidified film, the escaping solvent and also the non-solvent leave behind microchannels, which are then available in the coating for the transport of water vapor.
In a similar manner coatings can be produced by admixing salt powders with the polymer solution. After formation of the coatings the salt can be washed out with water leaving microcavities behind.
The perforation of compact foils using high-energy electron beams also leads to films which can be laminated and which possess high water vapor permeability. As a rule the volume of water vapor transported per unit of time and unit of area is more than 10 times as high with a water vapor-permeable coating as with a conventional compact film.
These methods have the disadvantage that they are chemically complicated, require complex apparatus or create considerable waste problems.
Recently a microporous coating has been prepared which does not require immersion bath technology. In this process, also known as evaporation coagulation, water is added to the solution of the polymer in a low-boiling solvent in an amount such that the resultant spreading-paste is just stable and capable of being applied to a suitable substrate. During evaporation the organic solvent escapes first. The continuously increasing proportion of water precipitates out the solid as in the immersion process and finally escapes on drying, leaving behind a microporous structure in the film.
All of these processes have the disadvantage that the microchannels or microcavities cause a weakening of the water vapor-permeable coating, i.e., the tensile strength and abrasion resistance clearly decrease in comparison with a compact film.
Therefore, there have been many attempts to produce water vapor-permeable coatings not only by physical methods, but also by chemical means. It has been proposed to produce such coatings using polyurethanes which partly contain water-soluble or hydrophilic structural components. DE-A-1,220,384 and 1,226,071 describe polyurethanes which contain glycols, diisocyanates and a difunctional hydrophilic structural component, which creates the water vapor permeability of the coating and is a macrodiol. In both cases polyethylene glycol having a molecular weight of about 1000 is used; the two applications differ only in the "vulcanization" mechanism, i.e., the subsequent crosslinking of the polyurethane elastomers.
Compact top coats of composite materials made from textile substrates and microporous coatings, such as those described in DE-OS 2,020,153, are also water vapor-permeable.
Polyethylene glycols can also be used as the diol component for the production of polyester polyols which are subsequently used to prepare polyurethane elastomers as disclosed in Japanese Patent Application 61/009,423. The resulting coatings possess good water vapor permeability and negligible water swelling.
Segmented polyurethane elastomers prepared from polyethylene glycols are also disclosed in EP-A 52,915.
Other organic hydrophilic components may also be added to polyurethanes in order to prepare water vapor-permeable coatings and composite materials. In particular, poly-.gamma.-methylglutamate can be blended with polyurethanes, chemically incorporated as a structural component, or grafted onto polyurethanes. DE-A 1,922,329 and 1,949,060 and JP-A 58,057,420 and 59/036,781 disclose these alternatives.
Recently, polyurethanes prepared from the above-mentioned polyethylene glycols have been of particular industrial interest for the production of water vapor-permeable compact coatings. These raw materials are inexpensive and commercially available. The polyurethanes and polyurethane-ureas obtained from these raw materials are also well known in principle. In contrast to the widely used polyurethane-ureas, which contain polyesters, polycarbonates or polyethers as macrodiols, these materials are capable of absorbing water, water vapor-permeable, and occasionally even strongly swellable or soluble in water.
For this reason hydrophobic polyols are admixed with the polyethylene glycols having hydrophilic characteristics. From these mixtures polyurethanes or polyurethane-ureas can be prepared which combine good water vapor permeability with good resistance to the effects of liquid water.
Two-component coating compositions containing ketoxime-blocked prepolymers, binuclear, cycloaliphatic diamines as crosslinking agent/curing agent and at most 15% by weight of solvent in the formulation are described in DE-A 2,702,090 and U.S. Pat. No. 4,248,756. In these systems the polyhydroxy compounds used to prepare the blocked prepolymers may contain polypropylene oxides and, where appropriate, ethylene oxide (EOx) units.
Blocked prepolymers, as described in EP-A 100,005, formed from polyhydroxy compounds which contain 20-100% by weight, preferably 40-80% by weight of EOx units may be cured to provide good water-vapor permeable coatings on textile substrates. However, the swelling of these coatings in water leads to the occurrence of pimple-shaped swelling phenomena when discrete drops of water are placed on the coating. For a textile consumer article these are not only an aesthetic fault, but a considerable technical defect.
It is an object of the present invention to provide coating compositions which may be used to prepare coatings which do not suffer from this defect.